Neutron generators



June 1939 J. E. BOUNDEN ETAL 3,443,314

NEUTRON GENERATORS Filed March 7, 1966 Sheet of 3 June 3, 1969 J.BQUNDEN EI'AL 3,448,314

NEUTRON GENERATORS Filed March 7, 1966 Sheet 2 of 3 June 3, 1969 J. E.BOUNDEN ET AL NEUTRON GENERATORS Sheet Filed March 7. 1966 United StatesPatent US. Cl. 31361 5 Claims ABSTRACT OF THE DISCLOSURE A neutrongenerator comprising a normally sealed off envelope, means to produce aplasma in gas within a portion of the envelope, a boundary electrodebounding said portion, from a further portion and having an aperture forthe extraction of ions from the plasma, an extractor electrode and atarget shield each having apertures in register with the boundaryelectrode aperture to allow passage of an ion beam and beingsuccessively spaced from the boundary electrode within said furtherportion, means to produce an axial magnetic field in the region of theboundary electrode aperture, and a target located behind the targetshield.

This invention relates to neutron generators and relates particularly togenerators comprising an envelope, normally sealed-off, in whichdeuterium and/ or tritium ions from an ion source are accelerated tostrike a target containing deutariurn and/or tritium to produce neutronsby the D-D and/or D-T reactions, the gas pressures in the ion-source andaccelerating portions of the envelope being equal. A generator of thistype is described in US. Patent No. 3,344,299 issued Sept. 26, 1967, toI. E. Bounden.

The generator described in this patent comprises a tubular glassenvelope, part of which forms the ion source and is encircled by an R.F. winding to produce a plasma therewithin. A frusto-conical extractorelectrode having a central aperture projects into the plasma, a beam ofions being Withdrawn through the aperture and accelerated towards atarget located at one end of the envelope. The target proper is shieldedfrom the accelerating field by a shield electrode. An output of up toneutrons per second is obtainable in either continuous or pulsedoperation.

To produce a larger output from this generator, the ion currentwithdrawn from the ion source through the aperture in the extractorelectrode would have to be increased. In theory this could be doneeither by increasing the RP. power to increase the density of the plasmaproduced, or by increasing the diameter of the aperture. The firstcourse is undesirable as likely to result in overheating of the glassenvelope, whilst to the second there are two objections. Firstly the ionbeam diameter is correspondingly increased, which means that theaperture diameter in the shield electrode has to be correspondinglyincreased; this is undesirable from the point of view of suppressingelectrons emitted from the target region, as will be explainedhereafter. Secondly, the increase in extractor aperture diameter wouldallow the accelerating field to penetrate farther into the ion sourceregion, leading to the risk of long-path electrical breakdown betweenthe shield and the interior of the ion source.

It has been found that the form of neutron generator provided by thepresent invention enables the ion beam current to be increased byapproximately an order of magnitude, whilst maintaining the beamdiameter at a value which does not necessitate an undesirable increasein the diameters of the electrode apertures.

According to the present invention a neutron generator comprises anenvelope, means for producing a plasma in gas within a portion of saidenvelope, a boundary electrode bounding said portion from a furtherportion of said envelope and having an aperture for the extraction ofions from said plasma, an extractor electrode and a target shieldsuccessively spaced from said boundary electrode within said furtherportion and having apertures in register with the aperture therein toallow passage of an ion beam, means for producing an axial magneticfield in the region of the aperture in the boundary electrode, and atarget located behind said target shield.

The plasma-producing means preferably comprises a RF winding encirclingsaid portion of the envelope, and the magnetic field producing means asolenoid coil encircling said envelope between said RF winding and saidboundary electrode.

Reference has been made to the suppression of secondary electronsemitted from the target. It is most important that back-streaming ofthese electrons, and of electrons formed by ionisation by the beam ofthe gas within the target shield, towards the ion source, should bereduced as much as possible because of the heat generated by thiselectron current. In the generator described in the aforementionedpatent, this was effected by maintaining the target slightly positivewith respect to the target shield, so that electrons emitted from thetarget were drawn back to the target. In the present generator thelatter arrangement was found to be inadequate when the current reachedonly two or three times that in the earlier generator, theback-streaming current then becoming sufiicient to damage the tube. Analternative arrangement has been used to remove this limitation onincreasing the ion current, allowing it to be increased about ten-fold.

Accordingly, the aperture in the target shield of the present generatoris very preferably of channel-like form, and an open-ended tubularsuppressor member is located Within said shield but electricallyinsulated from it, one end of said member encircling the target and theother terminating adjacent the target end of the channel in the targetshield.

These and other features of the present invention will now be described,by way of example, with reference to the accompanying drawings, wherein:

FIGURE 1 is a sectional elevation of a neutron generator embodying thepresent invention.

FIGURE 2 shows the potential distribution between the extractorelectrode and target of the embodiment of FIGURE 1.

FIGURE 3 is a sectional elevation of another embodiment of theinvention.

In this drawing one end of a sealed tubular glass envelope 1 isencircled by an RF winding 2 for exciting a plasma in gas within theenvelope. This portion of the envelope forms the ion source and isbounded by a boundary electrode 3 formed as a flat disc having a centralaperture 4 and sealed to the envelope 1. Electrode 3 is made ofmolybdenum for good heat conduction. Between winding 2 and electrode 3 asolenoid coil 5 encircles the envelope to produce an axial magneticfield in the region of aperture 4 and so intensify the plasma adjacentthereto. This increases the ion current density extractable from theplasma boundary for a given RF exciting power. The surface of electrode3 facing the plasma is thinly coated with vitreous enamel 6 to withinabout 0.005 inch of the edge of the aperture, leaving an enamel-freeregion 30, to shield the metal from the plasma in order to preventsputtering and also to minimise recombination of hydrogen isotope atomicions into molecular ions. Sputtering can also have a gas-absorbingeffect which is clearly undesirable in a sealedoff tube having a limitedgas content. Into aperture 4 is screwed an aluminum insert 34 having alip which extends over region 30, aluminum having a much lower atomicrecombination coeflicient than molybdenum or Nilo-K and also a muchlower sputtering ratio (i.e. atoms emitted per impinging ion). The lipof insert 34 contacts the plasma, thus maintaining it at the potentialof electrode 3 and also keying the plasma to the lip.

Spaced beyond boundary electrode 3 is a frustroconical extractorelectrode 7 brazed to an annular ring 8 sealed to envelope 1 and havingan aperture 9 into which is screwed an aluminum anti-sputtering insert35 similar to insert 34. Spaced in turn from electrode 7 is a targetshield 10 mounted on a metal tube 11 sealed to envelope 1 and having achannel-like aperture 12. Apertures 9 and 12 are seen to be in registerwith aperture 4.

Insulated from tube 11 by a tubular glass section 13 is a metal tube 14on the end of which is a flange 15 supporting the target 16 which is oferbium evaporated onto a molybdenum pressing, a re-entrant cavity 29being formed behind the target. The erbium layer is initiallyimpregnated with deuterium which is converted in operation to anapproximately 50/50 deuterium/tritium mixture by reason of thereplenisher 19 being initially charged with a deuterium/tritium mixturehaving an excess of tritium suflicient to make the total gas content ofthe envelope (i.e. including both replenisher and target) anapproximately 50/50 mixture. In operation the target is replenished in aknown manner by the action of the mixed ion beam. Also mounted on flange15 coaxially within shield 10 is a tubular suppression member 17, oneend of which encircles target 16 and the other end of which terminatesadjacent the target end of channel 12. In operation a suitable coolant,e.g. ICI Arcton 113 or Monsanto TAS 130, is circulated over the rearface of target 16 to remove the heat dissipated by the ion beam.

To the other end of the envelope 1 is sealed a metal tube 18 carrying anend-plate 21 on which are mounted the gas replenisher 19, a sealing-offtube 20, and a Pirani gauge (hidden behind tube similar in design to thecorresponding components described in the aforementioned patent. Alsofastened to plate 21, via support tube 22, is a copper disc 23 coatedwith evaporated aluminium to prevent sputtering by contact with theplasma.

Disc 23, which is cooled via aperture 24 in plate 21, acts as a stopperfor back-streaming electrons, both those produced by ion bombardment ofextractor electrode 9, and those from the region of target shield 10.Disc 22 is made of large diameter because the electron beam tends to beincreased in cross-section by the defocussing effect of the divergingmagnetic field produced by coil 5.

The drawing is approximately to scale, the external diameter of tubes18, 11 and 14 being 2 inches. As atready mentioned, electrode 3 andtarget 16 are made of molybdenum. Tubes 22, 11, 18, and 14, flange 15,tube 17, shield 10, and the extractor assembly 7/8 are made of Nilo-Kalloy. Envelope 1 is of Kodial glass.

Aluminium alloy guard rings 25 and 26 are clamped to disc 8 and tube 11respectively to prevent high electric stresses arising where thesecomponents are sealed to envelope 1. The ion-source region of envelope 1(as far as disc 8) including winding 2 and coil 5, are enclosed in afirst methylmethacrylate jacket through which cooling oil is circulated,and the region between ring 25 and flange 15 in a second similar jacketcontaining high-grade insulating oil.

In operation the envelope 1 is filled with the deuterium/ tritium gasmixture supplied by replenisher 19, the gas pressure being maintained atapproximately 15 X10- mm./Hg as measured by the Pirani gauge. A plasmais excited in this gas by applying R.F. power at 15 mc./s. to winding 2.Ions are extracted from the plasma through aperture 4 in boundaryelectrode 3 by a potential difference V of up to 5 kv. applied betweenelectrode 3 and extractor electrode 7, the latter being at earthpotential. The plasma takes up the potential of electrode 3 and ofelectron stopper 23, which are connected together externally, a curvedplasma boundary or cap forming over the plasma side of the aperture 4.Coil 5 produces an axial magnetic field of about gauss in the region ofaperture 4 to intensify the plasma, as already described.

At relatively low values of V part of the ion beam impinges on extractorelectrode 7, but as V is increased (keeping the plasma density constant)the entire beam is focused through aperture 9 as the plasma boundary orcap is pushed back by the adjacent electric field pro duced by V Theinitial ion bombardment of electrode 7 produces secondary electronswhich are accelerated back into the ion source through aperture 4 butare stopped by disc 23 as described.

The ion beam leaving extractor electrode 7 is further accelerated by themain accelerating field produced by the potential dilference V of -120kv. applied between electrode 7 and target shield 10, and impinges ontarget 16. Target 16 and suppressor'member 17 are maintained at apotential V of about +400 v. with respect to the target shield 10.FIGURE 2 shows by means of equipotential lines the potentialdistribution with electrodes 7, 10 and 17 held at 0, -100 kv. and 99.6kv. respectively, plotted by the electrolytic tank technique. It will beseen that the arrangement provides a relatively equipotential driftspace 27 extending from the target (not shown in FIGURE 2) to near theend of member 17, from where the potential rises sharply to a peakpotential of about 99.85 kv. in the region of point 28 before fallingsteadily towards 0 v. This peak or hump of about 250 v. around point 28acts as a trap for electrons formed within the shield 10 either by ionbombardment of the target or by ionisation of the gas molecules by thebeam ions, and inhibits their escape into the main accelerating field.

The use of a comparatively narrow channel 12 in shield 10, as comparedwith the simple apertures 4 and 9 in the other two electrodes, has theadvantage that the penetration of the main accelerating field into theshield is reduced. This in turn allows an electron-trapping hump ofgiven height to be produced with a lower bias voltage V than wouldotherwise be the case, which is advantageous since the bias voltage hasto be superimposed on the main accelerating potential of 100 kv., andhence should be as simple to generate as possible. It also has theeifect of locating the hump 28 closer to extractor electrode 7 thanwould otherwise be the case, which reduces the length of ion beam pathbetween electrode 7 and the hump, in which no trapping takes place. Theuse of too high a value of V may also tend to establish an undesirabledischarge between the suppressor tube and the target shield, which maycause sputtering of the latter.

As has already been mentioned, the relatively narrow, high-density, ionbeam produced in the present generator allows aperture 9 to be keptsmall, which reduces the penetration of the main accelerating fieldbeyond electrode 7 into the ion-source region and thus reduces the riskof long-path breakdown. (It may be noted that generators of the presenttype operate in that region of the Paschen curve, Where, for constantpressure, the breakdown voltage decreases as the gap length increases.)The narrow beam furthermore makes it possible to use the comparativelylong narrow channel in the target shield which, as discussed above,facilitates electron trapping.

Another advantage of the narrow ion beam is that its smaller angle ofdivergence on entering the target shield allows the target to be locatedrelatively far back from the shield aperture without the beamcross-section ex ceeding the target diameter when the beam impinges onthe target. This means that the neutrons are produced at a location moreremote from the main accelerating region of the tube and its associatedhigh-grade insulation, which greatly improves access to the target.Comparison with the generator described in U.S. Patent No. 3,344,299shows that in the present generator the target structure is much lessre-entrant, which makes it correspondingly easier to irradiate samples(e.g. for activation analysis) in the high-flux region immediatelybehind the target surface 16. As the re-entrant cavity also has to carrythe target coolant pipes, this is an important factor, especially if acompressed-air operated conveyor for rapid transfer of samples is to beincluded. In the embodiment of the invention shown in FIGURE 3 there-enter form of target is replaced by a target of which the surface isflush with the end of the tube, as hereinafter described.

A further advantage of the form of ion source used in the presentgenerator is that it enables the neutron output to be easily andaccurately controlled by varying the value of V rather than by varyingthe RF power supply to the plasma-exciting winding, as in the generatordescribed in the aforementioned patent. The latter system is ratherdependent on gas pressure fluctuations, which are difiicult to controlprecisely. Ease of output control is particularly advantageous inapplications requiring a modulated neutron output, e.g. in nuclearreactor experiments.

A yield of 9X10 neutrons per second has been obtained with theabove-described embodiment, filled solely with deuterium, under thefollowing opearting conditions:

Target voltage (V kv 120 RF power dissipated in plasma (approx) w 380Target shield current (I ma 6.0 Target current (I ma 5.0 Total tubecurrent (I ma 11 Suppressor bias (V v 440 Magnetic field (approx) gauss100 Extractor voltage (V kv 3.3 Extractor current (I ma 11 Interceptioncurrent (I ma The value of the target current I quoted above is notequal to the ion beam current striking the target, which is estimated tobe at least 8 ma. under the above conditions, because the target/suppressor assembly also collects electrons resulting from ionisation ofthe gas by the high energy ion beam.

The above neutron output is increased approximately 100-fold by fillingthe tube with a 50/50 deuterium/tritium mixture, instead of withdeuterium only.

In FIGURE 3, which illustrates another embodiment of the invention, thesecond and third digits of the reference numerals refer to portionswhich are correspondingly numbered in FIGURE 1. The portion of thegenerator to the left of ring 108 is the same as in FIGURE 1, but theremainder of the tube is modified as shown.

It has been found that there exists a tendency, when operating underhigh-voltage conditions, for electrical breakdown to occur between thetarget shield (FIG- URE 1) and the extractor electrode 7 along the innersurface 37 of the glass envelope 1 extending between ring 8 and tube 11.There is reason to believe that such breakdown may be initiated byirradiation of the. glass surface either by charged particlesoriginating from the ion beam, or by electromagnetic radiation (e.g.ultraviolet light from the ion source) or by X-rays due to highenergyelectrons striking the extractor or the back-stop and ion-source walls.Another possible cause is deposition on the glass of any materialsputtered by the beam from the outer region of shield 10 surrounding theend of channel 12.

Whatever the cause, it is found that this tendency to breakdown isreduced by a configuration of the extractor and shield electrodes whichserves to screen the glass surface from such influences. In FIGURE 3 theshield includes a cylindrical portion 131 which projects within acorresponding cylindlical portion 132 of electrode 107 so that there isa substantial overlap. The baflle eflect resulting from thisconfiguration prevents the portion 137 of the glass surface 101extending between these two electrodes from seeing the ion beam, or theapertures in electrodes 107 and 103 (and hence the plasma in the ionsource) or the end of channel 112.

To :preserve the electron-suppression and ion-optic characteristics ofFIGURE 1 with the extractor/shield configuration of FIGURE 3, the end ofsuppressor tube 117 adjacent channel 112 has a conical termination 136.This has the effect of keeping the suppression hump approximately thesame distance from the extractor as in FIGURE 1 and thus minimising theunsuppressed length of ion beam. A conical-ended suppressor tube has theeflect of reducing the height of the hump (for a given V but this eifectis countered by the increased effective length of channel 112, and thenet efiect is a hump of suflicient height for eifective suppression withV =400 v.

The tendency to breakdown between electrodes 110 and 107 is furtherreduced by two other modifications. Firstly the right-hand end ofportion 132, facing the shield 110, has an increased radius ofcurvature, as compared with FIGURE 1, to prevent the formation of localhigh electric fields. Secondly, portion 132 is stepped at 133 toincrease the clearance between the extractor and the envelope, whichreduces the potential gradient along the inner surface 137 of theenvelope. A similar step may be provided on the outer surface of theshield adjacent the envelope.

Mention has already been made of the advantage of reducing the degree ofre-entry of the target, to facilitate the irradiation of samples. In theembodiment of FIG- URE 3 the target 116 is seen to be a flat disclocated beyond flange 115.

What we claim is:

1. A neutron generator comprising an envelope, means for producing aplasma in gas within a portion of said envelope, a boundary electrodebounding said portion from a further portion of said envelope and havingan aperture for the extraction of ions from said plasma], an extractorelectrode and a target shield successively spaced from said boundaryelectrode within said further portion and having apertures in registerwith the aperture therein to allow passage of an ion beam, means forproducing an axial magnetic field in the region of the ape!- ture in theboundary electrode, and a target located beh nd said target shield.

2. A neutron generator as claimed in claim 1 wherein saidplasma-producing means comprises an RF winding encircling said portionof the envelope and said magnetic field producing means comprises asolenoid coil encircling said envelope between said RF winding and saidboundary electrode.

3. A neutron generator as claimed in claim 1 wherein the aperture in thetarget shield of the generator is of channel-like form, and anopen-ended tubular suppressor member is located within said shield butelectrically insulated from it, one end of said member encircling thetarget and the other terminating adjacent the target end of the chnanelin the target shield.

4. A neutron generator as claimed in claim 1 wherein the aperture in atleast said boundary electrode is provided with an insert made of a metalhaving a low sputtering ratio and a low atomic recombinationcoefficient.

5. A neutron generator as claimed in claim 1 wherein said extractorelectrode comprises a hollow cylindrical portion within which extends acylindrical portion of said target shield in concentric overlappingrelationship, said relationship being adapted to prevent the portion ofsaid envelope extending between the extractor electrode and the targetshield from being in direct view of said ion 15 beam and of theapertures in said electrode and shield.

References Cited UNITED STATES PATENTS 3,112,401 11/1963 Dorsten et a1.313-61 X 3,302,026 1/1967 Mallon et a1. 313-61 X 3,344,299 9/1967Bounden 250-845 X OTHER REFERENCES Activation Analysis: New Generatorsand Techniques Make It Routine by Meinke et al., Nucleonics, vol. 20,

10 No. 3, March 1962, pages 60 to 65.

JAMES W. LAWRENCE, Primary Examiner.

R. F. HOSSFELD, Assistant Examiner.

US. Cl. X.R. 250-845

